AbstractThis report describes the role of T cell‐derived lymphokines in induction of macrophage (Ma) tumor cytotoxicity. M tumor cytotoxicity was tested with the murine Ma‐like tumor PU5‐1.8. This MΦ‐like tumor reacts to stimulation with T cell‐derived lymphokines and shows tumor cytotoxicity comparable to resident peritoneal MΦ. The experiments demonstrate that immune interferon (IFN‐γ) secreted by T cell clones in limiting dilution microcultures was insufficient to induce MΦ tumor cytotox icity. Induction of MΦ tumor cytotoxicity by T cell‐derived supernatants with quantities of IFN‐γ undetectable in the IFN assay, however, was completely inhibited by an antiserum raised against recombinant IFN‐γ. Taken together, these results could be thus explained: (a) a T cell‐derived lymphokine distinct from but serologically cross‐reactive with IFN‐γ induces MΦ tumor cytotoxicity, and (b) IFN‐γ activity in supernatants positive for induction of MΦ tumor cytotoxicity escapes detection in the IFN assay but can still be inhibited by the anti‐IFN‐γ antiserum in the MΦ tumor cytotoxicity assay. The latter explanation requires positive evidence for another T cell‐derived lymphokine inducing MΦ tumor cytotoxicity together with IFN‐γ. This lymphokine was found in the supernatant of T cells in limiting dilution cultures and a long‐term T cell clone and was called Mγ cytotoxicity‐inducing factor 2 (MCIF2). MCIF2 synergizes with IFNγ induction of MΦ tum